Sleeve valves are used for reservoirs or the like. As shown in FIG. 1, the sleeve valve 24 comprises a valve body 18 secured to the upper wall 12 of a water chamber or tank 10 serving as a pressure reducing chamber, and a valve cylinder gate 20 having open ends and vertically slidably fitted in the valve body 18. The valve body 18 includes a lower end closed with a bottom cover 14 and a peripheral wall formed with a large number of discharge ports 16, while the valve cylinder gate 20 has an upper portion extending upward from the valve body 18 and upwardly or downwardly slidable in intimate contact with the inner surface of a high-pressure water inlet elbow 22 communicating with the reducing chamber. The gate 20 is coupled to an electric motor operator 26, which raises the gate 20 to the desired level relative to the bottom cover 14 to alter the number of the discharge ports 16 in communication with the interior and outside of the valve body 18 and to thereby control the flow of water into the reducing chamber.
When jetting out from the discharge ports of the valve body into the reducing chamber, the water produces an intense noise due to cavitation. It is known that the use of outwardly tapered ports 16 as shown in FIG. 7 noticeably reduces this noise and ensures a quiet operation, especially when the ports have a taper angle of about 11.degree.. However, great difficulties are encountered in producing a valve body formed with outwardly tapered ports in its peripheral wall. According to the usual method, the peripheral wall of the valve body is first drilled to form straight holes, and the holes are then reamed to a conical shape from inside the valve body. Thus a large number of ports 16 must be shaped one by one by manual operation which requires much labor and time.
Furthermore, the discharge ports are usually distributed uniformly over the entire area of the valve body peripheral wall, such that in a circumferential direction the ports are arranged in helical rows progressively ascending from the bottom cover 14 so as to continuously alter the outflow of water when the valve cylinder gate 20 is moved upward or downward. The results of experiments have revealed that the relationship between the opening percentage of the gate 20, h/h.sub. o, and the flow ratio, Q/Q.sub.o, can be represented by a curve A shown in FIG. 3, in which:
h: distance the gate is raised from the bottom cover, PA1 h.sub.o : maximum distance the gate is raisable from the cover, PA1 Q: rate of outflow of water from the valve body, and PA1 Q.sub.o : maximum rate of outflow of water from the body.
Note: Long pipe line is attached onto inlet elbow 22 and outlet pipe in this experiment.
It is seen that when the opening percentage is altered from 0 to 10, the flow ratio alters markedly from 0 to 85%, whereas even if the opening percentage is thereafter varied from 10 to 100, the flow ratio increases only by 15%.
In order to overcome this problem, the valve cylinder gate is usually raised gradually at a low velocity for the commencement of discharge of water, so that the sleeve valve requires a long time for opening.